article: One day, I happened to meet an Englishman in the street and soon we began to talk. As I was talking about how I was studying English, the foreigner seemed to be very surprised, gently shaking his head and saying "You don't say!"
  I was confused, and I thought, "Perhaps this is not a right thing to talk about." Then I said to him, "Well, shall we talk about the Great Wall? Have you ever visited it?"
  "Certainly! Everyone back home will laugh at me if I leave here without seeing it. The Great Wall is wonderful! " "Yes, it is one of the wonders in the world. And people of many countries have come to visit it." As I went on telling him more about it, he stopped me again, "You don't say!"
  I couldn't help asking, "Why couldn't I talk about it?"
  "Well, I didn't stop you talking about it," he answered, greatly surprised.
  "Didn't you say 'You don't say!'?" I asked again.
  Hearing this, the foreigner laughed loudly. He began to explain, "'You don't say!' means 'Really'. Perhaps you know little about English idioms ."
  Wow! How foolish I was! Since then I have been careful with English idioms.
question: The English idiom "You don't say!" means   _  .
options: Thank you Excuse me OK Really